1. Field of the Invention
The present invention relates to the field of downhole fluid analysis in hydrocarbon producing wells. More particularly, the present invention relates to a method and apparatus for using a chemometric equation to estimate fluid density, viscosity, dielectric constant, and resisitivity from flexural mechanical resonator data obtained downhole in a borehole during monitoring while drilling or during wireline operations.
2. Background of the Related Art
There is considerable interest in measuring density, viscosity, dielectric constant, resistivity and other parameters for formation fluids. It is particularly useful to perform these measurements downhole at reservoir conditions of high temperature and high pressure during formation sampling, producing or drilling operations. Numerous technologies have been employed toward the end of measuring fluid parameters such as viscosity downhole. U.S. Pat. No. 6,182,499 discusses systems and methods for characterization of materials and combinatorial libraries with mechanical oscillators. U.S. Pat. No. 5,734,098 (the '098 patent) discusses a method for monitoring and controlling chemical treatment of petroleum, petrochemical and processes with on-line quartz crystal microbalance sensors. The '098 patent invention utilizes thickness shear mode (TSM) resonators, which simultaneously measure mass deposition and fluid properties such as viscosity and or density of a fluid. U.S. Pat. No. 6,176,323 (the '323 patent) discloses drilling systems with sensors for determining properties of drilling fluid downhole. The '323 patent discloses a plurality of pressure sensors positioned at different depths to determine a fluid gradient. U.S. Pat. No. 5,741,962 (the '962 patent) discloses a method and apparatus for analyzing a formation fluid using acoustic measurements.
The '962 patent invention acoustically determines density and compressibility from acoustic impedance and sound speed. U.S. Pat. No. 5,622,223 (the '223 patent) discloses a method and apparatus for characterizing formation fluid samples utilizing differential pressure measurements. The '223 patent discloses an apparatus that provides two pressure gauges at different depths to determine density from a fluid pressure gradient. U.S. Pat. No. 5,006,845 describes an invention that uses differential fluid pressure at two depths to determine fluid density. U.S. Pat. No. 5,361,632 discloses a method and apparatus for determining multiphase hold up fractions using a gradiometer and a densiometer to provide a pressure gradient to determine fluid density. U.S. Pat. No. 5,204,529 discloses a method and apparatus for measuring borehole fluid density, formation density and or borehole diameter using back-scattered gamma radiation to determine fluid density.
Flexural mechanical resonators have been used in the laboratory for rapid characterization of large numbers of fluid samples. See L. F. Matsiev, Application of Flexural Mechanical Resonator to High Throughput Liquid Characterization, 2000 IEEE International Ultrasonics Symposium, Oct. 22-25, 2000 San Juan, Puerto Rico, incorporated herein by reference in its entirety; L. F. Matsiev, Application of Flexural Mechanical Resonator to High Throughput Liquid Characterization, 1999 IEEE International Ultrasonics Symposium, Oct. 17-20, Lake Tahoe, Nevada, incorporated herein by reference in its entirety; L. F. Matsiev, Application of Flexural Mechanical Resonator to High Throughput Liquid Characterization, 1998 IEEE International Ultrasonics Symposium, Oct. 5-8, 1998, Sendai, Miyagi, Japan, incorporated herein by reference in its entirety.
The use of mechanical resonators are described in U.S. Pat. No. 6,455,316 B1 which is incorporated herein by reference in its entirety; U.S. Pat. No. 6,393,895 B1 which is incorporated herein by reference in its entirety; U.S. Pat. No. 6,336,353 B2 which is incorporated herein by reference in its entirety; U.S. patent Publication No. 2003/0041653 A1 which is incorporated herein by reference in its entirety; U.S. patent Publication No. 2003/0000291 A1 which is incorporated herein by reference in its entirety; U.S. Pat. No. 6,401,591 B2 which is incorporated herein by reference in its entirety; and U.S. Pat. No. 6,6,528,026 B2 which is incorporated herein by reference in its entirety.
An example of a method and apparatus for determining down fluid characteristics using flexural mechanical resonators is described in the parent application, U.S. patent application Ser. No. 10/144,965 filed on May 14, 2002 entitled “Method and Apparatus for Downhole Fluid Characterization Using Flexural Mechanical Resonators.” (the '965 patent application). The '965 patent application describes a method or apparatus utilizing a flexural mechanical resonator to determine density, viscosity or other fluid properties in a downhole environment. An example of a suitable algorithm for use in association with the method and apparatus described in the '965 patent application is a Levenberg-Marquardt (LM) non-linear least squares fit. The LM fit uses an initial estimate for determination of a fluid parameter. If the initial parameter estimate is too far from the actual parameter values, the LM algorithm may take a long time to converge or fail to converge at all. Thus, there is a need for a method and apparatus for accurately estimating the initial parameter inputs for the LM algorithm in determining fluid parameters.